Method for mechanical packaging of electronics

ABSTRACT

Electronics mounted on a printed circuit board are housed within a high conductivity case with connecting pins extending therethrough. The case is filled with thermally conductive potting material to provide thermal conduction from the printed circuit board to the case. The case may be a conduit having open ends through which the printed circuit board is inserted or it may comprise a cover mounted to a base plate.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/503,874, filed Aug. 11, 2006 now U.S. Pat. No. 7,765,687 which claimsthe benefit of U.S. Provisional Application No. 60/707,605, filed onAug. 11, 2005.

The entire teachings of the above applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

There are standard package designs for certain types of military gradeDC-DC converters, filters and other electronic circuits such as thoseshown in FIGS. 1 and 2. The typical prior art for constructing thesecircuits and assembling them in their packages is shown in FIG. 3. Aring frame 301 (typically Kovar) is welded to a base plate 302(typically Molybdenum). A ceramic substrate 303 (typically Alumina,Aluminum Nitride, or Beryllium Oxide) is bonded to the base plate usingepoxies or solder alloys if the substrate is suitably plated. Electroniccomponents are soldered to the substrate which also contains thick andor thin film conductors. Most of the active microcircuits 304 (e.g.MOSFETs, diodes, control IC's) are soldered or epoxied to the substrateas bare die and wire bonded for connectivity. Magnetic devices 305 (e.g.transformer and inductors) are also surface mounted to the substrate.Input and output (I/O) pins 306 enter and exit the ring frame throughholes with hermetic glass seals 307 that also provide electricalisolation between the pins and the ring frame. The I/O pins are wirebonded to the substrate or connected using L shaped brackets. A thincover 308 (typically stainless steel) is seam soldered or welded to thering frame in a vacuum so that there are no opportunities forpenetration of external corrosive elements that would attack the baresilicon devices inside. The selection of all of the materials arecarefully made in order to minimize differences in thermal expansionproperties between the silicon of the active microcircuits, ceramicsubstrate, metal base plate, and glass seals.

SUMMARY OF THE INVENTION

The technology described above has been in use for decades, but it hasseveral disadvantages. The substrates are prone to fracture duringfabrication, mounting of electronic components, base plate attachmentand use in the field. The substrate mounting to the base plate permitsthe attachment of electronic components from one side only. The thickfilm conductors are expensive to produce and have inferior electricalconductivity compared to pure metals and can only be fabricated inlimited thicknesses, resulting in relatively high electrical resistanceswhich reduce the efficiency. The layer count on thick film substrates islimited and is typically only one or two layers. Interconnection betweencomponents is limited because of the low layer count and limitations onthe use of vertical integration access (vias). Magnetic devices 305(transformers and inductors) must be hand-assembled and surface mountedto the substrate. Handling and assembly of the bare siliconmicrocircuits requires the use of expensive clean room environments andspecialized handling and even dress of assembly personnel. The wirebonds that connect the microcircuits to the substrate are prone tofatigue or tensile failures under extreme vibration or accelerationconditions. The use of Molybdenum and Kovar result in a heavy package.The glass seals must be handled carefully to avoid fracture. All of thematerial systems as described above require assembly processes that arenot conducive to low cost, high volume manufacturing. Furthermore, thefinal assembly has a limited ability to withstand shock and vibrationstresses encountered under service conditions.

Disclosed are novel methods for the mechanical packaging of HighReliability (Hi-Rel) DC-DC converters, filters, or other such electroniccircuits for military or industrial applications.

There is a need for cost effective mechanical packaging for Hi-Relconverters and filter modules which enable advances in electronicperformance while maintaining physical integrity and reducing productweight. The advent of suitable plastic packaged electronic componentsnow permits their use in products that were typically produced inhermetically sealed enclosures. These plastic device packages providesufficient protection for the active microcircuits contained within sothat the exterior mechanical packaging itself is no longer required tobe hermetically sealed.

The mechanical packaging is considered to be the enclosure that protectsthe electronic components from the diverse environmental stresses to beencountered such as, but not limited to, thermal extremes, mechanicalshock and vibration, adverse chemical or sand exposures, etc. Thispackaging should be robust in order to provide the desired protection,but also be easy to produce and assemble in order to be cost effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first standard package design for a Hi-Rel DC-DCconverter or filter.

FIG. 2 illustrates a second standard package design for a Hi-Rel DC-DCconverter or filter.

FIG. 3 in a detailed illustration showing how a Hi-Rel DC-DC converteror filter is assembled and placed in a standard package.

FIG. 4 is a detailed illustration showing a new approach for how aHi-Rel DC-DC converter or filter can be assembled and placed in a newpackage design.

FIG. 5 is a detailed illustration showing another new approach for how aHi-Rel DC-DC converter or filter can be assembled and placed in a newpackage design.

FIGS. 6A and 6B illustrate the package design depicted in FIG. 5 fullyassembled.

FIG. 7 illustrates injecting a thermally conductive potting materialinto a new package design.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows.

New methods for the design of a Hi-Rel mechanical package and thesubsequent assembly of the circuit within it have been developed. Themethods provide lower cost and are easier to produce using standard,high volume production methods and equipment.

FIG. 4 shows an exploded view of one embodiment of the newconstruction/assembly. The basic principles of this method are asfollows.

The substrate is made from a printed circuit board 401 (PCB) fabricatedfrom layers of fiberglass and copper foil. PCB assemblies are inherentlymore mechanically robust than ceramic substrate assemblies. The PCBenables the use of pure metal, multiple layers (8-12 typical), and metalthicknesses up to 6 times those of typical thick film conductions. Theconductor layers are made from copper foil, and provide higherconductivity resulting in significantly lower interconnection resistanceand a resulting increase in efficiency of conversion. Planar windingscan be made as part of the PCB fabrication and used for the magneticdevices 402 (transformers and inductors), further improving efficiencywhile reducing fabrication and assembly costs. The PCB is also able tohave components mounted on both of its sides, improving componentdensity and enabling more functionality and performance. The multiplelayers of the PCB and the use of vias connecting various layers simplifyinterconnection. The components are assembled and soldered to eitherside of the PCB using standard Surface Mount Technology equipment and donot require clean room environments or handling. There need be nodelicate wire bonds in this construction.

The I/O pins 403 are soldered directly to the PCB using semi-circularattachment sites 404. The latter are formed as part of the PCBfabrication process. A plastic/insulating header 405 at each end of thePCB provides accurate positioning of the I/O pins within the package andprovides mechanical strain relief for the I/O pins. Locating features onthe PCB and header are used for alignment of the I/O pins to the PCBduring soldering.

The PCB/Header/Pin subassembly can optionally be conformal-coated withvarious material systems (e.g. Parylene C) for enhanced chemicalresistance and electrical isolation.

The base plate, ring frame, and cover from the prior art are allreplaced by a single extruded case 409 (typically made from Aluminum).This case is machined to provide the required mounting features 410. Thealuminum case is lighter and less expensive to produce than the priorsystem elements. The case is finished (e.g. hard anodized or zincchromated, a.k.a. Alodine) in order to provide corrosion resistance. ThePCB/Header/Pin subassembly slides into an opening at either end of thecase. The case has case locating holes 406 that align with thePCB/Header locating holes 407 in the PCB and headers.

Locking pins 408 (typically made from stainless steel) are press fitinto the locating holes on the case and lock the sub assembly in place.The locking pins have features that press fit into the PCB as well,providing connection from the case to the PCB, and subsequently to a‘Case Pin’ for electrical grounding and shielding purposes. The featuresmay be edges that cut into the case and PCB to provide a swage fit andenhance electrical contact to the PCB and case. The locking pins alsohave heads 412 on them so that they cannot vibrate loose outwardly. Thelocking pins are inserted from the bottom of the unit, with their headsset in recesses in the case, so that they are entrapped when the unit ismounted to a heat removal plate in the end application and thus cannotvibrate loose in the other (inward) direction.

As shown in FIG. 7, the case 409 also has a hole 711 in the bottom intowhich a thermal potting compound is injected from an injector 713. Thethermal compound (typically made from a mixture of RTV siliconeelastomer and Aluminum Oxide powder) provides a thermal path for heattransfer from the electronic components and PCB to the case for transferto the external cooling medium. The thermal compound is elastic andprovides attenuation for vibration and shock excitation without placingstress on the electronic components, enhancing the mechanicalreliability of the package. This construction enables a significantimprovement in the ability of the new package to withstand shock andvibration conditions that far exceed those that could be handled by theprior art. It should be noted that while the package itself and thepoints at which the leads exit the package have been demonstrated to becapable of withstanding vibration levels exceeding those of the priorart, care must still be taken to ensure that the electrical connectionsto the package are made in a manner consistent with survival underextreme shock and vibration conditions.

Another assembly with a different form factor has also been developed.FIG. 5 shows an exploded view of another method for packaging PCB basedconverters, filters, or other types of Hi-Rel circuitry.

The PCB assembly 501 has I/O pins 502 that are inserted into it duringSMT assembly and protrude from the bottom side. These I/O pins slip orpress fit into insulating bushings 503 that have previously beenpress-fit into an anodized aluminum base plate 504. The base plate canbe fabricated by a variety of conventional metal forming processes suchas casting, extrusion and/or machining processes. The bushings, whichcan be made using processes such as injection molding or machining,provide electrical isolation between individual I/O pins and the case. Acase ground connection is achieved by press fitting the pin directlyinto an appropriately sized hole in the base plate.

A cover 505 is installed onto the base plate to provide a completeenclosure. Overlapping seams are used to minimize openings for radiatedelectromagnetic interference (EMI). The cover is affixed to the baseplate using built in features such as a snap fit, screws, epoxy or acombination of such methods. As illustrated, tangs 512 on uprights 514snap into openings 510 in the cover to retain the cover on the baseplate. The interior region is then filled with a thermally conductivematerial via a hole located on one of the six external surfaces. Lockingpins may also be added in the same manner as described above to providecase grounding. The fully assembled product 601 is shown in FIGS. 6A and6B.

The resulting mechanical packages made using the above methods arelighter and less expensive to produce than the prior art and can easilybe assembled using simple, standard, cost effective techniques. This newpackaging method has been tested under shock, vibration and otherstringent environmental conditions as described in MIL-STD-810 andMIL-STS-883, and it meets or exceeds the applicable requirements.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims

What is claimed is:
 1. An electronics package comprising: a four sidedcase; a printed circuit board within the case, wherein the printedcircuit board is inserted into an open end of the four sided case andsupports connecting pins coupled to and extending from opposite ends ofthe printed circuit board; locking members locking the printed circuitboard to the case, each locking member having a head recessed in thebottom of the case to be mounted to a heat removal plate; and thermallyconductive potting material filling the case and providing a thermalpath from the printed circuit board to the case.
 2. The package asclaimed in claim 1, further comprising a header coupled to each end ofthe printed circuit board to support the connecting pins.
 3. The packageas claimed in claim 1, wherein the printed circuit board isconformal-coated.
 4. The package as claimed in claim 3, wherein theprinted circuit board is conformal-coated with Parylene C.
 5. Thepackage as claimed in claim 4, wherein the thermally conductive pottingmaterial comprises a mixture of elastomer and ceramic.
 6. The package asclaimed in claim 5, wherein the ceramic is aluminum oxide.
 7. Thepackage as claimed in claim 6, wherein the elastomer is a siliconeelastomer.
 8. The package as claimed in claim 1, wherein the thermallyconductive potting material comprises a mixture of silicone elastomerand aluminum oxide powder.
 9. The package as claimed in claim 1, whereinthe heat removal plate is to be mounted external to the case, so thateach locking member is entrapped between the case and the heat removalplate when the case is mounted to the heat removal plate.
 10. Thepackage as claimed in claim 1, wherein each locking member makeselectrical connection between the printed circuit board and the case.11. The package as claimed in claim 10, wherein the locking member hasfeatures that cut into the printed circuit board and case.
 12. Thepackage as claimed in claim 10, wherein the components are mounted usingsurface mounted technology.
 13. The package as claimed in claim 1,further comprising mounting electronic components on both sides of theprinted circuit board.
 14. The package as claimed in claim 1, whereinthe printed circuit board supports a DC/DC converter.